The invention concerns a radio frequency (RF) resonator for the resonant transmission and/or reception of RF-signals at a desired resonance frequency into or out of a sample in an investigation volume within a homogeneous magnetic field B.sub.0 of a nuclear magnetic resonance (NMR) apparatus, wherein the RF resonator comprises superconducting components.
A radio frequency resonator of this type is per se known in the art from U.S. Pat. No. 5,585,723.
One of the most important requirements in NMR spectroscopy is the achievement of high sensitivity for the NMR signal, i.e. a high signal-to-noise ratio (S/N ratio).
The magnitude S of the signal is primarily dependent on the geometrical construction of the resonator, and on how close the resonator surrounds the sample. The temperature of the resonator thereby plays a less important role.
In contrast thereto, the noise voltage of the resonator is a strong function of temperature. The resonator comprises inductive and capacitive components which are in resonance at the desired frequency. The noise voltage N is produced in the RF loss resistance R.sub.V of the resonator and comprises the noise in the capacitive portion of the resonator (quite small and can practically be neglected) and in the dominating inductive portion. It is thereby this latter noise portion which is most important for the resonator noise and this component is dependent on both the temperature T as well as on the temperature-dependent RF loss resistance R.sub.V (T): EQU N .alpha..sqroot.T.multidot.R.sub.V (T)
The temperature dependence of the S/N-ratio is given in the following equation: ##EQU1##
A reduction in the temperature T of the resonator leads to an increase in the S/N ratio due to two effects. First of all, due to the lower temperature T in the denominator of the above expression and, second of all, due to the loss resistance R.sub.V (T) which is likewise smaller at lower temperatures.
It is therefore advantageous to cool the resonator down to a very low temperature e.g. to temperatures in the range of 4K to 20K. If one chooses superconducting material to construct the inductive portion of the resonator, particularly good results are achieved, since, with a superconductor, the RF loss resistance R.sub.V (T) is substantially smaller than with a normally conducting metal such as copper. In this manner, the S/N ratio, as given in the above equation, can be very high.
Modern NMR spectroscopy measuring methods use, almost exclusively, RF pulses for the excitation of the magnetic spin system with a subsequent Fourier transformation. Excitation is normally effected using the same resonator with which the NMR signal is subsequently detected. It is therefore important, after generation of the RF pulse, for the resonator to be free of current flow as quickly as possible in order to optimize detection of the NMR signal.
However, resonators having very small losses also have very narrow resonance lines and therefore very long excitation pulse decay times. A certain time t.sub.1 % must pass before the resonance current decays e.g. to 1% of its initial value: EQU t.sub.1 %=9.21 L/R(T)
where L is the resonator inductance.
This equation clearly shows that the smaller the loss resistance R(T) of the resonator the longer the decay time.
In order to be able to apply modern measuring methods, modern day applications utilize very short RF excitation pulses (hard pulses) which can assume values on the order of 10 .mu.s and less. If one notes that the product between the pulse duration of the excitation pulse and the field amplitude at the location of the sample must have a defined optimum value, these short excitation pulses lead to very large field amplitudes and therefore to extremely high electric RF currents in the resonator. These must first decay sufficiently before switching to detection. Unfortunately, delayed detection results, however, in distorted base lines and distorted lines in the NMR spectrum. With superconducting resonators, additional loss resistance can therefore be advantageous to reduce the decay time of the excitation pulse. These loss resistances can, however, not be too large, since this would lead to unacceptable deterioration of the S/N-ratio. For this reason, these methods are only applicable if the S/N-ratio is already trimmed to a very high value e.g. through use of a suitable geometry for the resonators to provide a particularly high fill factor.
Since construction of superconducting NMR resonators is a new development direction in NMR, there are not a large number of relevant publications. The above mentioned U.S. Pat. No. 5,585,723 represents the present prior art. The superconducting resonator (see FIGS. 16a and b) is introduced as a complete resonant system, i.e. with both its inductive as well as capacitive components, on a flat crystal plate 18a which is coated on one side with a superconducting material 19a and is mounted close to the sample 5.
In this case, the resonator must be large compared to the diameter of the sample, in order that the produced RF field be sufficiently homogeneous in the vicinity of the sample. An improvement in this regard can be achieved through the combination of two identical resonators 19a and 19b which are disposed as Helmholtz-resonator pairs to the left and the right of the sample 5. One thereby not only achieves a more homogeneous RF field but also a stronger coupling into the sample, i.e. an improved fill factor.
Such a Helmholtz-resonator configuration is a resonant-capable system having two prominent resonant frequencies: an upper one with the currents in both resonators flowing in opposite directions, and a lower one in which the currents flow in the same direction. For NMR applications, the lower resonant frequency must be utilized since only this mode produces the desired homogeneous RF field of the location of the sample. The resonators are normally inductively coupled to pass the NMR-signal to the detection system of the NMR spectrometer.
Modern day systems use high-temperature superconductors (HT superconductor) as the superconducting material such as e.g. YBCO whose electrical properties are weakly dependent on the static magnetic field B.sub.0. These HT superconductors are generally deposited as thin layers on a crystal plate so that the crystal grains from which these conductors are formed are all oriented in the same direction only in this manner can the HT superconductor achieve the best electrical properties. The surface of the crystal plate serves as a substrate upon which the crystalline superconducting layer is introduced and forced to assume the orientation of the crystal plate. It is therefore advantageous to use a crystal plate whose crystal structure is as close to that of the superconductor as possible. If this plate is to be also used as a dielectric for the capacitive components of the resonator, it should also have good dielectric RF properties. All these requirements are e.g. fulfilled by both LaAlO.sub.3 and sapphire crystals. It can also be advantageous if these plates are good thermal conductors in order to guarantee a better cooling of the superconducting layer. Sapphire also fulfills this requirement.
The use of HT superconductors has, however, an additional advantage. Due to their high critical temperature in the vicinity of 100K, there is a larger temperature region available within which the good superconducting RF properties of the resonator are effective. This allows for increased flexibility when adjusting the operating temperature.
Such crystal plates are only available today in the form of flat plates and therefore the resonator must also have a flat structure. The plate can be cooled to a cryogenic temperature using a cooled helium gas flow having a temperature below 20K.
The geometric configuration of modern conventional resonators has the serious disadvantages described below.
The resonator configurations comprise one or two individual resonators which are built on one or two crystal plates, with each individual resonator being a complete resonance system. If one assumes the optimum configuration, namely a Helmholtz-resonator (see FIGS. 17a/b), then one would dispose this Helmholtz-resonator as closely as possible around the sample to achieve as high a fill factor as possible. Since, however, the Helmholtz-pair consists essentially of two flat structures which are poorly adjusted to the cylindrical geometry of the sample, one nevertheless fails to achieve a large fill factor. This is particularly evident in the corner regions of the two resonators which are relatively far from the sample. Only the horizontal transverse connections 20 are in close proximity to the sample and are more strongly coupled thereto.
The horizontal transverse connections 20 have an additional very serious disadvantage. They are not parallel to the B.sub.0 -field, but perpendicular thereto and this is something which should be avoided if it all possible, since it leads to a deterioration of the homogeneity of the B.sub.0 -field. In order to limit this deterioration, it is necessary for the superconductor to be very narrow, i.e. to be made with as little material as possible. However, the maximum possible RF current below which a linear dependence between the RF current and the RF field obtains, is thereby reduced as is the associated maximum possible RF field. In order to generate a particular NMR flip angle, it is therefore necessary to tolerate longer pulse times which leads to undesirable spectra for a plurality of NMR experiments.
An additional disadvantage is caused by the vertical longitudinal connections 21 of the resonator which, for reasons of space, are very close to an RF shielding 6. They therefore produce eddy currents in the shields and associated RF losses which transform back into the resonator. In this manner, although the decay time of the excitation pulse is advantageously reduced, the signal-to-noise ratio simultaneously deteriorates. Since, however, the fill factor of such a Helmholtz configuration is already poor, the negative influence of the RF losses on the S/N-ratio is particularly noticeable.
In contrast thereto, it is the purpose of the present invention to create an RF resonator for NMR applications having the above mentioned features which, despite the use of superconducting materials, facilitates a significantly higher fill factor, with the homogeneity of the B.sub.0 field not being deteriorated and the S/N-ratio being particularly high.